FIG. 5 is a diagram showing a conventional switching power supply circuit described in xe2x80x9cHigh-Frequency Isolation UPS with Novel SMRxe2x80x9d (IECOM ""93, pp. 1258-1263, (1993)). Such a conventional switching power supply circuit includes a first series circuit including diodes D1 and D2 connected in series, a second series circuit including MOSFETs Q1 and Q2 connected in series, and a third series circuit including MOSFETs Q3 and Q4 connected in series. The conventional switching power supply circuit also includes a snubber circuit SN connected in parallel with the first through third series circuits.
A first AC input terminal U is connected to the common connection point of the diodes D1 and D2. A primary winding section P1 of a transformer T1 having center taps is connected to the common connection point of the MOSFETs Q1 and Q2. The other primary winding section P2 of the transformer T1 is connected to the common connection point of the MOSFETs Q3 and Q4. The center tap between the primary winding sections P1 and P2 of the transformer T1 is connected to a second AC input terminal V.
The secondary winding sections S1 and S2 of the transformer T1 are connected to an end of a capacitor C5 via diodes D3 and D4, respectively. The center tap between the secondary winding sections S1 and S2 of the transformer T1 is connected to the other end of the capacitor C5. DC output terminals P and N are connected to the capacitor C5.
When the transformer T1 (the primary winding sections P1 and P2) is short circuited by switching ON the MOSFETs Q1 and Q3 while the AC input voltage is positive, the current of a reactor L1 increases. When the MOSFET Q3 is switched OFF in the state described above, the reactor current flows through the primary winding section P1 from the MOSFET Q1, feeding electric power to the capacitor C5 via the secondary winding section S1 and the diode D3.
When the transformer is short circuited again by switching ON the MOSFET Q3, the reactor current increases. When the MOSFET Q1 is switched OFF subsequently, the reactor current flows through the MOSFET Q3. The reactor current flowing through the MOSFET Q3 excites the primary winding section P2 of the transformer T1, and electric power is fed to the capacitor C5 via the secondary winding section S2 and the diode D4.
By repeating the operations described above at a high frequency, the AC input voltage is insulated and converted to DC electric power by the transformer T1. The insulated and converted DC electric power is output via the diodes D3, D4 and the capacitor C5. When the AC input voltage is negative, the conventional switching power supply operates in the same manner as described above by switching ON and OFF the MOSFETs Q2 and Q4.
The conventional switching power supply circuit employs four MOSFETs Q1 through Q4, a reactor L1, and a snubber circuit SN. Since it is necessary for the conventional switching power supply circuit to incorporate four driving circuits, each driving any of the MOSFETs Q1 through Q4, the size of the conventional switching power supply circuit is large and the cost thereof is high. Since there exits certain time points where the single-phase AC input voltage is zero, the energy fed to the load is interrupted, causing large ripple voltages on the load. For obviating this problem, it is necessary for the capacitor C5 on the load side to have a sufficiently large capacity. Therefore, the size of the conventional switching power supply circuit is further enlarged and the cost thereof increased even further.
Accordingly, there is a need for a switching power supply circuit that at least reduces the capacity of the capacitor on the load side to reduce its size, weight, and cost. The present application addresses this need.
The present invention relates to a switching power supply circuit. The switching power supply circuit can have a transformer, a pair of diodes, which can form a first circuit, a pair of switching devices, which can form a second circuit, a pair of capacitors, which can form a third circuit, a snubber capacitor, and first and second AC input terminals.
The transformer can have a primary winding with a center tap and a secondary winding. The pair of diodes are connected in series, as are the pair of switching devices and the pair of the capacitors. The first AC input terminal is connected to both diodes or the common connection point of the two diodes. The second AC input terminal is connected to the center tap of the primary winding. The snubber capacitor, the pair of diodes, the pair of switching devices, and the pair of capacitors are connected in parallel. One end of the primary winding is connected to both switching devices or the common connection point of the switching devices. The other end of the primary winding is connected to both capacitors or the common connection point of the pair of capacitors.
The switching power supply circuit can further include a rectifying circuit, which can be connected to the secondary winding of the transformer, and a switching means for converting a single-phase AC input voltage to a high-frequency AC voltage. The transformer can insulate the high-frequency AC voltage. The rectifying circuit can rectify the insulated high-frequency AC voltage to feed DC electric power to a load.
The switching power supply circuit can further include at least one capacitor connected in parallel with at least one of the switching devices, wherein the at least one capacitor allows at least one of the switching devices to execute a zero-voltage switching. The switching power supply circuit can adjust an ON-OFF duty ratio of one of the switching devices to regulate the current of the AC input voltage and the operating frequency of the other of the switching devices to regulate the DC electric power.